title: Probing the role of aromatic residues at the secondary saccharide-binding sites of human salivary alpha-amylase in substrate hydrolysis and bacterial binding.
abstract: Human salivary alpha-amylase (HSAmy) has three distinct functions relevant to oral health: (1) hydrolysis of starch, (2) binding to hydroxyapatite (HA), and (3) binding to bacteria (e.g., viridans streptococci). Although the active site of HSAmy for starch hydrolysis is well-characterized, the regions responsible for bacterial binding are yet to be defined. Since HSAmy possesses several secondary saccharide-binding sites in which aromatic residues are prominently located, we hypothesized that one or more of the secondary saccharide-binding sites harboring the aromatic residues may play an important role in bacterial binding. To test this hypothesis, the aromatic residues at five secondary binding sites were mutated to alanine to generate six mutants representing either single (W203A, Y276A, and W284A), double (Y276A/W284A and W316A/W388A), or multiple [W134A/W203A/Y276A/W284A/W316A/W388A; human salivary alpha-amylase aromatic residue multiple mutant (HSAmy-ar)] mutations. The crystal structure of HSAmy-ar as an acarbose complex was determined at a resolution of 1.5 A and compared with the existing wild-type acarbose complex. The wild-type and the mutant enzymes were characterized for their abilities to exhibit enzyme activity, starch-binding activity, HA-binding activity, and bacterial binding activity. Our results clearly showed that (1) mutation of aromatic residues does not alter the overall conformation of the molecule; (2) single or double mutants showed either moderate or minimal changes in both starch-binding activity and bacterial binding activity, whereas HSAmy-ar showed significant reduction in these activities; (3) starch-hydrolytic activity was reduced by 10-fold in HSAmy-ar; (4) oligosaccharide-hydrolytic activity was reduced in all mutants, but the action pattern was similar to that of the wild-type enzyme; and (5) HA binding was unaffected in HSAmy-ar. These results clearly show that the aromatic residues at the secondary saccharide-binding sites in HSAmy play a critical role in bacterial binding and in starch-hydrolytic functions of HSAmy.
authors.affiliation: Department of Oral Biology, University of Medicine and Dentistry of New Jersey, C-634, MSB, 185 South Orange Avenue, Newark, NJ 07103, USA.
authors.forename: Chandran
authors.initials: C
authors.lastname: Ragunath
authors.forename: Suba G A
authors.initials: SG
authors.lastname: Manuel
authors.forename: Venkat
authors.initials: V
authors.lastname: Venkataraman
authors.forename: Hameetha B R
authors.initials: HB
authors.lastname: Sait
authors.forename: Chinnasamy
authors.initials: C
authors.lastname: Kasinathan
authors.forename: Narayanan
authors.initials: N
authors.lastname: Ramasubbu
mesh_terms: D020909:Acarbose / Q000737:chemistry; D000596:Amino Acids / Q000378:metabolism*; D001422:Bacterial Adhesion; D001425:Bacterial Outer Membrane Proteins / Q000302:isolation & purification / Q000378:metabolism; D001665:Binding Sites; D002241:Carbohydrates / Q000737:chemistry*; D002942:Circular Dichroism; D017886:Durapatite / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D050505:Mutant Proteins / Q000378:metabolism; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D055291:Salivary alpha-Amylases / Q000737:chemistry* / Q000378:metabolism*; D013213:Starch / Q000378:metabolism; D055672:Static Electricity; D054773:Streptococcus gordonii / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D020349:Surface Plasmon Resonance; D014961:X-Ray Diffraction
chemical_list: C112818:AbpA protein, Streptococcus gordonii; D000596:Amino Acids; D001425:Bacterial Outer Membrane Proteins; D002241:Carbohydrates; D050505:Mutant Proteins; D009844:Oligosaccharides; D013213:Starch; D017886:Durapatite; D055291:Salivary alpha-Amylases; D020909:Acarbose